2.10.1 Milk intake, calf growth and importance in diet
Recent research into dairy heifer nutrition has shown that feeding greater amounts of milk and feeding milk for longer periods of time increases the growth rate of calves and studies have shown that increasing energy and protein intake during the milk- feeding period increases calf growth rate and first lactation milk yield (Khan et al., 2011; Soberon et al., 2012; Bach, 2012; Margerison et al., 2013). These higher growth rates of dairy calves were typically, with the exception of Margerison et al. (2013), achieved by increasing MR (Bartlett et al., 2006; Davis Rincker et al, 2011; Soberon et al., 2012) or by increasing whole milk (WM) feeding levels from 10 % to 17.9 % (Jasper and Weary, 2002) and up to as much as 20 % of BW (Khan et al., 2007 a; Khan et al., 2011). However, increasing milk feeding volume was found to reduce starter intake at weaning to half that of calves offered milk at the equivalent to 10 % of BW (Jasper and Weary, 2002; Cowles et al., 2006; Raeth-Knight et al., 2009), which often results in BW loss and difficulty for the calf following weaning (Keil et al., 2000; Jensen, 2003) and more importantly increases calf rearing costs (Jasper and Weary, 2002).
Restricting WM and MR feeding levels has been found to increase starter feed intake (Jasper and Weary, 2002; Margerison and Downey, 2005) and increase the rate of rumen development and growth at weaning (Suarez-Mena et al., 2011). A relationship has been clearly demonstrated between milk feeding and starter intake (Terré et al., 2007; Raeth-Knight et al., 2009) and the intake of cereal based calf starter feeds was greater when milk feeding levels were restricted, typically to the equivalent of 10 % of BW (Margerison and Downey, 2005; Khan et al., 2007 a, b). Starter intake also shows a sharp increase when milk was withdrawn completely (Jasper and Weary, 2002). Moreover, dairy calves fed greater amounts of MR, from 562 up to 1,358 g/day (Cowles et al., 2006) and from 423 up to 704 g/d (Bascom et al., 2007), rather than greater volumes of milk diets have been found to have greater growth rates, especially when fed MR that had a greater crude protein concentration
(up to 31 % of DM) and when less of the gross energy (4.9 to 5.2 kcal/g) in the MR was derived from fat (Fat: 15 to 21 % of DM) (Cowles et al., 2006; Bascom et al., 2007). These changes in MR composition have been found to achieve greater growth rates and energy retention, along with lower fat and greater lean tissue deposition (Diaz et al., 2001; Tikofsky et al., 2001) in MR fed calves. In these studies, maintaining the energy concentration of MR was achieved by the addition of lactose (from 42.9 to 43.4 % of DM) (Diaz et al., 2001; Tikofsky et al., 2001; Hill et al., 2010). In older calves, between 2 to 13 weeks of age, lactose can be replaced by cereals (Huber et al, 1968; Toullec, 1989; Margerison et al., 2013) as a non-fat energy source, due to the development of pancreatic function (Guilloteau et al., 2009).
2.10.2 Concentrated starter feed intake and importance in diet
Calves offered low milk allowance were observed to consume more concentrated feed, which could be important for stimulating early rumen development (Nielsen et al., 2008). Volatile fatty acids (VFA), particularly butyrate and propionate, are the most important substrates stimulating rumen papillae growth (Coverdale et al., 2004; Suárez et al., 2006; Castell et al., 2012). However, feeding only concentrated feed resources can lead to a reduction in rumen pH, hyperkeratinisation, and decreased ability to absorb nutrients (Castell et al., 2012).
Warner et al. (1956), showed rumen development in calves offered varying diets (Figure 1). Calves offered milk only (Photo 487) was not physiologically different from that of new born calves (Photo 504), due to nutrients bypassing the rumen through the rumo-reticular (oesophageal) groove and resulting in limited rumen development, whereas calves offered cereal grain and milk (Photo 483) had dense papillae population, longer papillae length and width, which were stimulated by VFA production, from cereal grain fermentation. Finally, when calves are offered milk and forage (Photo 490), the rumen had less well developed papillae and the rumen wall was thicker and more muscular, due to the physical structure of high fibre forages.
Figure 2.3: Photographic comparison of the inside of the rumen on different diets (Warner et al., 1956) * *photograph 504 is the rumen of a newborn calf; 487 is the rumen of a calf offered only milk for thirteen weeks; 483 is the rumen of a calf offered milk and ad-libitum grain for 13 weeks; 490 is the rumen of a calf offered milk and ad-libitum hay for 13 weeks
2.10.3 Forage sources and importance in diet
In European countries, at least 100 g/hd/d of a fibrous food must be available from two weeks of age, increasing to 250 g/hd/d at 20 weeks of age (Phillips, 2004). If the diet is pre-mixed ration, the forage in the diet should be between 10 to 25 % and the fibre source must be palatable and stimulate rumination (Castells et al., 2012). The feeding of forage to very young calves, before three weeks of age, has been associated with reduced concentrated feed intake, impaired rumen papillae development and poor forage digestion (Castells et al., 2012). Delaying feeding forages until after concentrates have been consumed can aid in rumen development
(Margerison and Downey, 2005). VFA’s stimulate the microbial population which in
turn decreases the rumen pH level and increases the availability of VFA’s for
absorption which in turn drives rumen epithelial growth (Heinrichs and Lesmeister, 2005). Feeding forages causes an increase in rumen pH which decreases VFA availability due to the increased fibre content and particle size (Heinrichs and Lesmeister, 2005).
Forages and fibrous feeds stimulate saliva production. Saliva contains urea and minerals, important for rumen pH, microbial growth and development, and rumination, which can result in a reduction in cross-sucking and non-nutritive sucking (Moran, 2002; Coverdale, 2004; Castells et al., 2012). Forages are especially important in the muscular development of the rumen wall (Coverdale, 2004; Castells et al., 2012) and aid in the growth and development of the rumen papillae, mainly due to the physical abrasion of the papillae and break down of the outer layer of keratinisation (Castells et al., 2012). High fibre diets and diets consisting of a mash have been found to result in the production of large amounts of butyric acid, which is important for rumen development. Forages, alone, are more likely to result in the production of acetate and insufficient butyrate for an adequate rate of papillae development and as such feeding cereal based concentrated starters that increase butyrate and propionate production should be fed feed alongside forages (Coverdale et al., 2004; Khan et al., 2011; Castells et al., 2012).
The importance of the diets fibre level and the physical form of the diet for calves in avoiding bloat and parakeratosis and instigating rumination (Porter et al., 2007). The feeding of high amounts of forages increased the length of time spent feeding, which resulted in the behavioural needs of the cattle being satisfied (Keil and Langhans, 2001) to a greater extent. Providing pasture, as forage for calves, greatly reduced the performance of oral behaviours, such as licking objects in the environment and grooming of others (Phillips, 2004). Whereas calves offered straw ate more forage and concentrated starter resulting in a greater growth rate (Phillips, 2004) and rumination time, compared to calves offered pasture hay.
Pelleting of feeds has been shown to have a positive impact on palatability of feed, but has a higher rate of passage due to decreased digestibility (Porter et al., 2007). The higher rate of passage through the rumen was explained by the smaller particle size and therefore the shorter digestion time which affects rumination (Porter et al., 2007).